1. Field of the Invention
The present invention relates to an exposure apparatus. In particular, the present invention relates to an exposure apparatus for transferring, successively in a stitching manner, a pattern formed on a mask through a projection optical system onto a plurality of areas existing continuously on a substrate.
2. Description of the Related Art
Those hitherto known as exposure apparatuses for producing liquid crystal displays (LCD) include an apparatus of the so-called step-and-repeat system in which a predetermined area on a glass plate for liquid crystal (hereinafter referred to as "plate", if necessary) is exposed with a part of a pattern formed on a reticle (mask), and then the plate is subjected to stepping over a certain distance, followed by repeated exposure with other parts of the pattern on the reticle again and again.
In order to achieve precise positional adjustment for the reticle and the plate, the exposure apparatus as described above has a reticle stage for moving the reticle in a plane perpendicular to an optical axis of a projection optical system, and a plate stage for moving the plate in a plane perpendicular to the optical axis of the projection optical system. The exposure apparatus further has a magnification controller for controlling the projection magnification of the projection optical system so that a pattern on the reticle is projected at a predetermined magnification onto the plate. The control for the projection magnification is required for the following situation.
In order to produce a liquid crystal display, it is necessary that a pattern formed on a reticle is projected onto a pattern previously formed on a plate so that the former is overlaid on the latter. However, an area on the plate (shot area) with a pattern having been transferred upon exposure for a previous layer occasionally undergo stretching due to stretching of the plate caused by processes applied after the exposure for the previous layer. When overlay exposure is performed under such a condition, if the projection magnification of the projection optical system is left fixed, then it is impossible to precisely overlay and transfer a pattern formed on the reticle onto the shot area. Accordingly, in such a situation, the projection magnification is adjusted by using the magnification controller.
In the conventional exposure apparatus as described above, the projection magnification has been controlled as follows. When a secondly applied resist film (resist film applied after exposure and development for a firstly applied resist film) or a resist film applied thereafter is exposed, positions of alignment marks formed on the plate are detected prior to exposure for each layer. Degrees of stretching of the plate in two-dimensional directions are calculated on the basis of a result of the detection. Specifically, a scaling in the x direction (designated as .gamma..sub.x) and a scaling in the y direction (designated as .gamma..sub.y) are calculated provided that the plate stage is movable in the x direction. A simple mean value m of them (m=(.gamma..sub.x +.gamma..sub.y).times.1/2) is set as a magnification correction value, and the magnification controller controls the projection magnification of the projection optical system on the basis of the magnification correction value m. Thus the projection magnification has been hitherto automatically corrected.
However, the conventional technique as described above often failed to precisely overlay the pattern both in the x and y directions, as based on the correction for the projection magnification of the projection optical system by using the magnification correction value calculated as the simple mean value of the scalings in the x and y directions.
Namely, as shown in FIG. 7, an inconvenience as described below has often occurred. It is assumed that a shot area originally occupied an area S illustrated by dotted lines has expanded to an area S' illustrated by solid lines upon exposure for a second layer or followings as a result of expansion of a plate caused by downstream processes. If the projection magnification is corrected on the basis of the simple mean value m of the scalings in the x and y directions, the pattern is transferred only in an area narrower than each shot area in the x direction, while the pattern is transferred beyond each shot area in the y direction, as illustrated by hatched line sections in FIG. 7.
In the exposure for producing liquid crystal displays, the requirement for the overlay accuracy and the continuity of patterns at a stitching section to patch mutually adjoining shot areas is not necessarily at the same level in the x and y directions. A preference is afforded in many cases to the pattern continuity and the overlay accuracy in any one of the x and y directions depending on a direction of the pattern. For example, when a thin film transistor (TFT) is formed, source and drain layers are subjected to overlay exposure on a gate layer having been formed by first exposure. In this procedure, it is necessary to lay emphasis on a direction along which the source and drain layers are overlaid. In such a case, it is clear that no satisfactory result is obtained by using the conventional technique as described above.